Many chemical reactions are equilibrium limited. When chemical equilibrium is reached, the rate of forward reaction equals the rate of the reverse reaction, thus imposing a limit on the degree to which reactants can be converted to products. Therefore, the chemical equilibrium acts to limit the efficiency of the reaction. Although the amount of product available from an equilibrium-limited reaction is restricted, the products formed may be sufficiently valuable for one to attempt the reaction in spite of the limitation. Industry has used several techniques to maximize the amount of product formed. For example, the most common approach has been to further process a fixed bed reactor effluent containing an equilibrium mixture of reactants and products in order to separate the products from the reactants and recycle the reactants to the fixed bed reactor. The separated product may be collected, and recycling the reactants avoids waste. However, when using this approach, the reactant recycle volume is often large and costly, and appreciable amounts of reactants may be needed to form relatively small amounts of product. An example of using a reactor in series with separation techniques and recycling unconverted reactants may be found in U.S. Pat. No. 5,113,024, where diisopropyl ether is formed from water and propylene. This patent teaches that unconverted propylene in the reactor effluent may be separated from the product and byproducts by extraction and recycled to the reactor.
Another approach has been to make use of the knowledge that the separation of one or more products from the reaction mixture will upset the chemical equilibrium and allow additional product formation. Reactive distillation is a common example of this approach. With reactive distillation, the reaction is conducted under distillation conditions so that as product is being formed, it is rapidly separated from the reactants by distillation, thereby shifting equilibria to favor the yield of products. The reaction of methanol and acetic acid to form methyl acetate has been carried out using this technique; see U.S. Pat. No. 4,435,595.
This same reaction has been disclosed in U.S. Pat. No. 5,405,992 as being conducted by reactive chromatography which similarly separates the reaction products from each other in order to shift equilibria to favor the yield of products. A general discussion of reactive chromatography may be found in, Vaporciyan, G. G.; Kadlec, R. H. AIChE J. 1987, 33 (8), 1334-1343; Fish, B. B.; Carr, R. W. Chem. Eng. Sci. 1989, 44, 1773-1783; and Carr, R. W. In Preparative and Production Scale Chromatography; Ganetsos, G., Barker, P. E., Eds.; Chromatographic Science Series Vol. 61; Marcel Dekker: New York, 1993; Chapter 18. U.S. Pat. No. 5,405,992 teaches using a simulated moving bed containing a solid or mixture of solids that is effective both to catalyze an esterification reaction of an alcohol and a carboxylic acid, and to separate the products from each other through adsorption of at least one product in order to increase the yield of a product ester. Simulated moving bed reactive chromatography units may require substantial capital investment, and the present invention maintains the equilibria shift to favor the yield of products as provided by reactive chromatography, but allows the size of the simulated moving bed reactive chromatography unit to be substantially reduced, thereby reducing the capital investment. The present invention operates with a fixed bed of catalyst serially connected to a simulated moving bed reactive chromatography unit. In the fixed bed of catalyst, the reaction proceeds to partial completion or, in a preferred embodiment, until chemical equilibrium is reached. The reaction mixture is passed to the simulated moving bed reactive chromatography unit where the products are separated from the reactants causing additional product formation.
Fixed catalyst beds have been combined with other types of catalyst beds such as in U.S. Pat. Nos. 5,354,451, 5,211,838, 5,190,639, and 3,864,240 where a fixed catalyst bed or multiple fixed catalyst beds are used with a moving catalyst bed to perform catalytic reforming. Fixed catalyst beds have also been combined with simulated moving adsorbent beds, as in WO 92/07097 where a fixed catalyst bed, used to produce glucose and fructose from sucrose, is combined with a simulated moving bed to separate the glucose and fructose. However, applicants are the first to realize that a fixed catalyst bed and a simulated moving bed reactive chromatography unit can be successfully combined to improve the effectiveness of the fixed catalyst bed and to improve the cost efficiency of the simulated moving bed reactive chromatography unit.
The two-stage approach of the present invention provides enhancements over both a single stage fixed bed process and a single stage simulated moving bed process. For example, in the two-stage process of the invention, performing a portion of the reaction in a fixed catalyst bed reduces the size of the simulated moving bed as compared to the size needed where the entire reaction takes place in a simulated moving bed. Since fixed catalyst beds are less expensive to build, operate, and maintain than more sophisticated reactors such as moving bed or simulated moving bed reactors, the cost of the fixed bed is more than offset by the available savings through the reduction of the size of the simulated moving bed. Also, any catalyst poisons in the system will likely foul the catalyst in the fixed bed and not reach the catalyst in the simulated moving bed. Catalyst replacement or regeneration in a fixed bed is rudimentary compared to catalyst replacement or regeneration in a simulated moving bed. The simulated moving bed part of the combination provides the capacity to take the reaction largely to completion while due to equilibrium limitations the fixed catalyst bed alone would provide only partial reaction.